Many types of printers have been developed. The best printer to use in a particular application depends on factors such as the printer's relative cost, reliability, availability, speed, recording medium, and marking techniques. However, when direct marking on a recording medium is required, drop-on-demand printers are an appropriate choice.
Numerous kinds of drop-on-demand printers are either available or under development. For example, nozzle-based ink jet printers which emit ink through a small nozzle or orifice have been available for some time. Despite the work that has gone into developing these printers, they remain subject to various problems such as nozzle clogging; high production costs, which is at least partially a result of the difficulty in producing the nozzles; and image smearing, a result of using slowly drying ink to reduce clogging. While various solutions to these and other problems have been implemented or proposed, experience suggests that nozzle-based ink jet printers are not optimum.
In view of the above, other types of drop-on demand printers have been proposed. Far example, Kohashi in U.S. Pat. No. 4,383,265, issued 10 May 1983, disclosed an electroosmotic ink recording apparatus potentially usable for drop on demand printing. The '265 patent teaches the wicking of ink over an electrode using electroosmosis and the subsequent inducing of ink to jump from the wick onto a recording surface by using coulomb forces developed via a second electrode behind the recording medium While the technology found in the '265 patent may avoid some of the problems with nozzle-based printers, its teachings have not achieved wide spread use.
In any event, other drop-on-demand print technologies are being developed. One such technology of special interest to the present invention involves the use of capillary waves, specifically as taught in U.S. Pat. Nos. 4,719,476 and 4,719,480, respectively entitled "Spatially Addressing Capillary Wave Droplet Electors and the Like," and "Spatial Stabilization of Standing Capillary Surface Waves." Both patents issued to inventors Elrod, Khuri-Yakub, and Quate on 12 Jan. 1988, and both are hereby incorporated by reference.
U.S. Pat. No. 4,719,480 teaches methods and devices for spatially stabilizing the crests of capillary waves on the free surface of a liquid, such as ink, while U.S. Pat. No. 4,719,476 discloses methods and devices emitting droplets from the crests of capillary waves. In particular, U.S. Pat. No. 4,719,476 discusses ejecting droplets from the crests using acousticaily induced secondary capillary waves, heaters, laser beams, and ions. These techniques for ejecting droplets from capillary waves may not be optimum.
What is needed are easily implemented methods and devices for inducing droplets to be ejected from the crests of capillary waves. Such methods and devices would be particularly useful in drop-on-demand printers.